1. Field of the Invention
The present invention relates to a silica glass crucible used for pulling a silicon single crystal.
2. Description of the Related Art
The Czochralski method (the CZ method) is widely used for growing a silicon single crystal. In this method, a polycrystalline silicon material is filled into a crucible, heated and melted so that silicon melt is accommodated, then a seed crystal is brought into contact with a surface of the above-mentioned silicon melt. As the crucible is rotated, this seed crystal is rotated in an opposite direction and pulled upwards at the same time, to form a single crystal in the lower end of the seed crystal.
As the crucible for manufacturing this silicon single crystal, a silica glass crucible is used conventionally. This silica glass crucible has a container-shape and a two-layer structure, in which the outer layer of the container is formed with an opaque layer and the inner layer is formed with a transparent layer. The outer opaque layer includes a large number of bubbles and is formed of natural silica glass whose purity is low compared with synthetic silica glass, but which is excellent in heat resistance. The inner transparent layer is formed of a natural silica material or a synthetic silica material. Since the synthetic silica glass has an advantage that there are few impurities and a yield of dislocation-free single crystal (hereinafter referred as DF yield) is good, a so-called synthetic silica glass crucible is popular in recent years.
However, in the case where a single crystal is pulled up using the silica glass crucible as mentioned above, there is a technical problem that rotation of the silicon melt and the seed crystal or immersion of the seed crystal causes the surface of the silicon melt to vibrate (hereinafter referred to as melt vibration). In other words, there is a problem that when this melt vibration occurred, it becomes difficult to seed the seed crystal so that the pull-up cannot be started easily, and that the crystal is more likely to be dislocated when being pulled, and the DF yield decreases.
In order to solve such a problem, patent document 1 discloses a silica glass crucible (not shown) which is provided with a large number of recesses in the inner periphery of the crucible (initial melt line zone or upper region including this zone) with which the surface level of the silicon melt (melt line) is in contact at the time of starting the pull-up of the single crystal. According to the crucible as disclosed in patent document 1, protruding parts are formed at the end of the above-mentioned large number of recesses so that local surface tension of the surface of the silicon melt is increased to control the melt vibration of the silicon melt.
[Patent document 1] Japanese Patent Publication (KOKAI) No. 2005-272178
However, as for the crucible in which recesses are conventionally formed in the inner periphery of the crucible, since there are a large number of fine recesses exposed at the inner surface of the crucible, a cleaning fluid, such as hydrofluoric acid, does not sufficiently enter into the inside of the recesses in the final etching process so that the impurities or particles which are produced at the time of forming the recesses are not removed sufficiently. As a result, there is a problem that as the inner surface of the crucible is eroded by the silicon melt at the time of pulling the single crystal, the impurities and particle are released into the silicon melt, dislocation occurs in the silicon single crystal, leading to a need for meltback which is a process of melting the crystal into the silicon melt, and to reduction in the DF yield.
Further, the silica glass crucible is arranged and controlled to be perpendicular within a carbon crucible in a pull-up apparatus. However, it is not always to be arranged to be exactly perpendicular. When it is arranged to be inclined, a recess-forming zone which is formed at the inner periphery of the crucible and has a height of 0.5 mm-10 mm is not fully matched to a position (melt line) of the surface of the silicon melt over the whole circumference at the time of starting the pull-up, and it cannot be said that the melt vibration control effect is certainly sufficient.